Customarily, gaskets have been fabricated from asbestos or asbestos composite sheets, owing to the strength asbestos imparts to a composite material, to its relative low cost, good high temperature characteristics, and adaptability in the manufacturing process. It is now believed, however, that exposure to asbestos may be unhealthy, and that the use of asbestos should be carefully controlled and even prohibited in certain circumstances. Therefore, substitutes for asbestos useful in the manufacture of sealing members would be of interest.
A material found to have suitable properties for use in the manufacture of sealing members is graphite felt, a sheet material. Graphite felt is prepared from natural or synthetic graphite by a process which utilizes the crystalline layer structure of graphite, see e.g., Shane, et al., U.S. Pat. No. 3,404,061, "Flexible Graphite Material of Expanded Particles Compressed Together," and Yamazoe, et al., U.S. Pat. No. 4,234,638, "Composite Graphite Sheets." The carbon atoms in each layer are bonded in a hexagonal array but between the layers there are only weak van der Waals forces. It has been found that due to this weak interlayer force, the distance between the graphite layers may be opened up or expanded. This is accomplished by treating graphite powder with an expanding agent having strong oxidizing properties, such as nitric acid, sulfuric acid, mixtures of nitric and sulfuric acid, perhaloacids such as HClO.sub.4 and the like, followed by rapid heating. Under these conditions, the graphite particles expand or exfoliate in an accordion fashion to as much as one hundred times their original volume. The fluffy, exfoliated powder thus formed is pressed between platens or rollers to form a sheet having the general characteristics of a felt, web, paper, strip, foil or the like.
In the making of gaskets, this graphite felt may be substituted for asbestos composite sheets by die cutting to the appropriate shape. However, since pressed exfoliated graphite is generally weak and brittle, the edges of a sealing member formed therefrom must be laminated and reinforced with metal. Further, the sheet method generates a certain amount of waste as portions of graphite felt must be excised for every sealing member formed. Although this waste can be avoided by pressing the exfoliated powder in a die of the desired shape, the extreme reduction of volume--about one hundred fold--required to go from powder-loaded die to final gasket thickness presents equipment problems.
Moreover, in typical applications, compression molded exfoliated graphite will function adequately at temperatures up to about 200.degree. C. However, it will be destroyed by oxidation above 450.degree. C. when used in air and at about 600.degree. C. when used in a sealed environment.
Consequently, it would be desirable to develope a sealing member which is easy and economical to manufacture and is sufficiently strong to be used with reduced or with no metal reinforcing.
It is therefore an object of this invention to provide a sealing member made from graphite which has improved properties.
It is another object of this invention to provide a sealing member that is useful in dry atmospheres and in a vacuum.
It is another object of this invention to provide a sealing member containing graphite which is stable in air and over a range of temperatures.
It is another object of this invention to provide a sealing member containing graphite which has superior sealing performance due to higher strength and greater edge holding properties.
It is still another object of the present invention to provide a composite sealing member of intercalated graphite and a metal powder, a ceramic powder and/or a polymer.
It is still another object of this invention to provide a more efficient method of manufacture for graphite sealing members requiring simplified dies for pressing.
It is a still further object of this invention to provide a graphite sealing member requiring reduced amounts of metal reinforcement.